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it is reflected from side to side, again and again, till it reaches the tub, and there we see it shining brightly. It is a prisoner in the water, and follows it down into the tub. When you put your hand in the falling water, you see that it is lighted brightly, and yet the stream by comparison is rather dark.

If it were pure, distilled water, it would hardly be visible. As it is full of floating specks and motes, each of these reflects light, and these cause the water to appear full of light.

This fountain of fire is a charming experiment for a school, and its double lesson makes it as interesting as it is beautiful.

THE WATER-LANTERN.

Fig. 24 represents the water-lens used in the last experiment but two. The water-lens stands in the wooden box containing the mirror, and at the back of the box is a wooden slide holding a horizontal shelf at the top. This slide has a long slot cut in it, and, by means of a bolt and nut fastened at the back of the box, it can be made fast to the box in any desired position. This slide is 16 inches (40.6 centimetres) long, 5 inches (12.7 centimetres) wide, and inch (19 millimetres) thick ; and the slot cut in it extends nearly the whole length.

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The shelf on the top is 7 inches (17.8 centimetres)

long, and 5 inches (12.7 centimetres) wide, and has a hole, 34 inches (8.3 centimetres) in diameter, cut in the centre. The iron bolt and nut must go through the back of the box, and must have a washer wide enough to cover the slot in the slide. A few inches below the bolt a block of wood is fastened to the back of the box in the slot of the slide, to serve as a guide in raising and lowering the slide carrying the lens. In the hole in the shelf rests a large watchglass, or shallow dish, about 4 inches (10.1 centimetres) in diameter. The plano-convex lens used in our experiments in projection may be here used in place of the watch-glass. On each side of the shelf are two upright wooden arms, and between them is placed a looking-glass 7 inches (17.8 centimetres) long, and 4 inches (10.1 centimetres) wide. To hold this mirror in place, screws may be put through the top of uprights into the frame, so that it will hang suspended, and turn freely up or down.

This apparatus can be made for about $3.20, the woodwork costing $1, the two mirrors $1.50, the two lenses costing 70 cents; and when it is ready for work it will be a fine lantern suitable for projecting large pictures upon a screen. Place the lantern before the heliostat, so that the full beam of light will be reflected from the mirror upward through the glass bowl and the watch-glass. Fill each of these with clear water, and then place the swinging mirror at the top at an angle of 45°. Hang up a large screen of white cotton cloth or sheet in front of the lantern, and from 15 to 40 feet (4.57 to 12.20 metres) from the lantern. On this screen will appear a circle of light projected from the lantern. The sunlight from the mirror is refracted in the large water-lens and brought to a focus. It is again refracted in the small glass of water, and is reflected by the mirror on the screen. Get a piece of smoked glass, and trace upon it some letters, and then lay it on the water-lens with the top (upper side of the writing) toward the screen: immediately the letters will appear on the screen, in white on a black ground. If the projection is not distinct, loosen the nut at the back of the box, and move the wooden slide up or down till the right focus is obtained.

This water-lantern may now be used for all the work performed with ordinary magic lanterns. Place a sheet of clear glass over the large lens to keep the dust out of the water, and then lay common lantern-slides on this as in a magic lantern.

The most simple slides for such a lantern can be made by laying thin paper over engravings or drawings, and tracing the picture with lines of holes pricked with a pin. In the lantern such a paper slide will show the lines of the picture in dots of light on a dark ground. Another way is to write or draw on sheets of smoked glass. A curious effect may be made by placing the smoked glass in the lantern and writing upon it, upside down and backward, when the letters will appear to grow out in big white characters on the dark screen, and afford much amusement to all who see it. Of course, the film of smoke will easily rub off, and each scratch and finger-mark will be shown on the screen, and the work is often dirty and troublesome; but it has the advantage of being quickly done, and, if the picture is not right, it can be rubbed out and another put in its place.

A better kind of slide may be made by drawing with a needle on sheets of gelatine. Sheets of gelatine, 18 inches (45.7 centimetres) square, can be bought for 35 cents, either pure and transparent or in a variety of colors. Lay a piece of this on an engraving, and trace the picture, drawing, map, or outline, with the point of a large needle—do not press very hard on the gelatine; a mere scratch is enough—and in the lantern every line and dot will be visible, in black upon a white or colored ground. To preserve these sheets of gelatine, put them between sheets of glass, and bind them together with paper pasted over the edges.

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